This invention relates to thermoluminescent material and associated uses and associated process for making thermoluminescent material.
"Thermoluminescence" is often used to mean any luminescence appearing in a material due to the application of heat. As used hereinafter "thermoluminescence" refers to the emission of light due to the freeing of trapped electrons by application of thermal energy.
Many solids that contain luminescent centers often contain one or more types of centers that trap electrons. Upon application of suitable wavelengths of light or application of x-rays, such solids produce excited electrons. Upon removal of energizing light, x-rays or other radiation, the excited electrons may be trapped at an energy level higher than their ground state. If the depth of the trap (that is, the amount of energy required to release the electron from the trap) is large and the temperature is low, the electron will remain trapped for a long time. However, if the trap depth is sufficiently low, the solid may receive sufficient thermal energy to free such trapped electrons even at room temperature. An electron freed from a trap will return to its ground state and emit a photon.
Thermoluminescence may be characterized as a type of phosphorescence. Phosphorescence is generally considered a delayed light emission after the removal of a source of exciting energy used to cause electrons to assume an energy state higher than their ground level. Such "phosphorescence", often called "afterglow", will be used herein to refer to delayed emission having a duration which becomes shorter with increasing temperature.
Although various thermoluminescent materials have been heretofore developed, practical uses of such materials has been limited due to numerous limitations in the materials themselves. For example, some materials require a relatively high energy input (light used to excite electrons) in order to trap a sufficient number of electrons to provide a relatively low afterglow effect. On the other hand, materials which are more efficient at trapping electrons may have trap depths which preclude room temperature level thermal energy from releasing the electrons and causing the light emission. Although the latter materials may be useful for other phenomena, such trap depths are not desirable or suitable for thermoluminescent material. Materials which have a very low electron trap depth do not store electrons sufficiently securely to allow an afterglow effect to last for an appreciable time after removal of the light or other energy used to excite the electrons.
The above and other limitations were disadvantages of previously available thermoluminescent materials, which have greatly limited the possible applications for using the phenomena of thermoluminescence Additionally, the range of possible uses for thermoluminescent material has been limited by the restrictions in form of various thermoluminescent materials.